Hydrothermal alteration in the Berlin Geothermal Field (El Salvador): new data and discussion on the natural state of the system

The Berlin geothermal field (Republic of El Salvador), located about 110 km ESE of the city of San Salvador, is a water-dominated system with temperature close to 300°C in the production zone. The geothermal wells at Berlin intercepted calc-alkaline more or less hydrothermally altered lava flows and pyroclastic rocks. This study presents data on hydrothermal alteration, together with their distribution with depth, microprobe analyses of hydrothermal minerals and geochemistry of altered rocks. The stability of some minerals under natural state (i.e. before exploitation and reinjection) and present state reservoir conditions have been evaluated both using mineral saturation indices and activity vs. temperature diagrams. This information contributes to the reconstruction of the thermal and chemical characteristics of the field and their temporal evolution and they, therefore, play an important role for the correct evaluation of the geothermal resource and for the future exploration strategies. Four mineral alteration zones, occurring at increasing depths, have been distinguished on the basis of the abundance and appearance of characteristic hydrothermal minerals. The hydrothermal phases may either replace the igneous minerals, or fill cavities or fractures crosscutting the rock. The shallowest alteration zone is characterized by the presence of relatively low-temperature minerals, such as saponite, montmorillonite, heulandite etc.. At greater depth, hydrothermal phases, such as epidote, wairakite, prehnite, hydro-garnets typical of higher temperature conditions, occur. Quartz, calcite and chlorite are widespread in most of these zones. Geochemical analyses show that fluid-rock interaction caused an increase of Ca and LOI, no variation of Mg and Fe and a decrease of Na and Si in an altered core sample with respect to unaltered rock. Whereas, K was totally leached out from the altered rock. The shape of the deepest alteration zones gives a record of a past natural state condition (not the most recent) and contrast with present-day thermal feature, suggesting that significant thermal variations occurred from the time of alteration formation to present-day. Recent natural state and present state conditions of the reservoir fluid were computed by using geochemical data of the present-day discharged fluids. Temperature and the CO2 content decreased, in most of the wells, evolving from the natural state of the system to the present state conditions. These variations can be explained with variable degrees of degassing due to isoenthalpic or sub-isoenthalpic boiling. Mineral saturation indices indicate that wairakite is unstable under both present and natural state conditions. Log[Ca2+]/[H+]2) vs. temperature diagram indicates that epidote is generally not stable under both present and natural state conditions, its stability depends mostly on the CO2 partial pressure. This explains why epidote compositions are not correlated to the temperature. Chlorite appears to be stable under both present and natural state conditions and its composition varies with temperature. The AlIV content in this mineral from different wells shows, in fact, a fairly good correlation with temperature following the equation: T(°C) = 47.238 x AlIV + 175.77. This equation, therefore may be used for temperature estimates for the Berlin geothermal field